Several lines of evidence suggest that substance P (SP) and excitatory amino acids (EAAs) mediate pain transmission at the spinal cord level. Our past studies provide evidence that kainic acid evokes the release of SP from the dorsal spinal cord and this release is, in turn, potentiated by the N-terminal portion of SP. This interaction may reflect a possible mechanism underlying pain transmission and be related to the "wind up" phenomenon observed electrophysiologically. We will test the hypothesis that (1) pain is mediated by the release of both SP and EAAs, and (2) that these two systems interact synergistically. EAAs cause the release of SP from primary afferent C-fibers. SP in turn potentiates the effect of EAAs via the N-terminal portion of SP at a receptor that is unique from that of the neurokinin receptor. The accumulation of SP of N- terminal metabolites during chronic pain alters SP and EAA activity along nociceptive pathways. On the other hand, decreased SP released during chronic opioid administration may lead to changes in SP- and EAA- mediated nociceptive activity that are reflected in opioid withdrawal symptoms. [1] We will use electronmicroscopic techniques together with immunocytochemistry and anterograde labelling to determine whether EAA receptors are located on SP-containing primary afferents. [2} We shall construct dose-response curves for the ability of [D-Pro(2), D- Phr(7)]SP(1-7) (D-SP(1-7)), a SP N-terminal analog that inhibits (3)H- SP(1-7) binding, to inhibit the effects of SP(1-7) on SP- and EAA- induced behavior and on opioid-induced antinociception. [3] We will determine the effect of SP(1-7) on "wind up" and on EAA-induced firing of antidromically identified nociceptive dorsal horn neurons that project to the thalamus. [4] Using in vivo microdialysis of the spinal cord of freely moving rats, we will determine (a) the role of SP(1-7) in EAA-induced release of SP-like immunoreactivity from primary afferent C- fibers, and (b) the role of EAAs and SP(1-7) in noxious stimulation- induced SP release using NMDA, non-NMDA and SP(1-7) antagonists. [5] We will determine whether nociceptive involves a synergistic interaction between SP and EAA receptors by measuring the antinociceptive effects of EAA and SP antagonists.